A Monoclonal Antibody Specific to a Song System Nuclear Antigen in Estrildine Finches

Transcription

1 Neuron, Vol. 31, , August 30, 2001, Copyright 2001 by Cell Press A Monoclonal Antibody Specific to a Song System Nuclear Antigen in Estrildine Finches Eugene Akutagawa and Masakazu Konishi 1 Division of Biology California Institute of Technology Pasadena, California Summary using mrna probes against rare cdna clones made from whole canary telencephalons, we instead generated monoclonal antibodies against homogenates of carefully excised song system nuclei from male zebra finches. The song system is a discrete, interconnected group of neurons in the brain of songbirds that mediates singing behavior (Nottebohm, et al., 1976). Our results demonstrate an antibody whose staining pattern is highly specific to the song system of some estrildine finches (family Estrildidae of Africa and the South Pacific, including the zebra, Bengalese, and spice finches). Labeled cells within the song system are cleanly compartmentalized and show very little overlap with other, unrelated brain areas. Since our antibody does not stain the song system of canaries, it may be that estrildine finches represent an exception to Clayton s postulated combinatorial principle of molecular organization of the brain. In any case, our results clearly show that unique molecules do exist to identify specific brain areas that are functionally linked. Moreover, the highly restricted temporal and spatial expression pattern among certain song nuclei points to shared molecular mechanisms that were involved in the formation of their respective pathways. This paper describes a monoclonal antibody that recognizes a molecule whose expression is mostly restricted to some of the forebrain areas that control singing behavior in adult estrildine species studied, including the zebra, Bengalese, and spice finches. When the song system displays extreme sexual dimorphism, as in these species, antibody staining occurs only in the male s song nuclei. However, protein expression is identical in both sexes of estrildine finches, in which females also have a well-developed song system. Canaries appear to lack the protein, but it can be induced in female zebra finches by early estrogen treatment. Antibody staining patterns in the zebra finch show that the protein s expression is developmentally regulated to coincide with the abrupt increase in the volume and cell size of the male s or the estrogen-treated female s song system. Results Introduction Production and Characterization of the Antibody Genetic and molecular approaches have brought about Monoclonal antibodies were produced in our facilities much progress in understanding the mechanisms of using homogenates of one of the song nuclei in the adult body plan development (Lumsden and Krumlauf, 1996), male zebra finch, RA (robust nucleus of the archistriaaxonal path finding (Tessier-Lavigne and Goodman, tum), while utilizing the cyclophosphamide immunosup- 1996), neuronal differentiation (Tanabe and Jessell, pression method (Matthew and Sandrock, 1987; Ou, et 1996), and the establishment of correct neuronal con- al., 1991). After screening close to one thousand clones nections (Goodman and Shatz, 1993). So far, however, individually on fixed brain sections of adult male zebra there are only a few examples of molecules that have finches, only one clone displayed a staining pattern that been shown to be preferentially expressed in the com- was highly specific to the song system. The initial mother ponents of distinct neural circuits, such as the visual or clone, designated as 11A12, was later divided into five auditory pathways, when they form functional entities. subclones: C2, H4, F5, G2, and D4. All of the subclones One reported exception is LAMP (limbic system-associ- displayed the same pattern of staining in the zebra finch ated membrane protein) that governs selective neural brain and were isotyped as mouse IgMs. Our antibody growth and axon targeting in the limbic system of rats showed a highly localized pattern of binding, specific (Levitt, 1984; Zacco et al., 1990; Pimenta et al., 1995). for only a few clusters of neurons that are grouped to- However, the results are compromised somewhat by the gether to form distinct nuclei. Very few, and randomly occurrence of some overlapping distributions to other scattered, labeled cells were observed throughout the unrelated parts of the brain. This is further compounded rest of the brain. At the cellular level, immunostaining by the lack of a clear anatomical definition of the limbic was restricted to the nucleus of neurons; hence we will system. The paucity of references on this subject indi- refer to the antigen that our antibody recognizes as the cates that such molecular signatures are either very rare, Song system Nuclear Ag (SNAg). difficult to isolate, or both. For example, when Clayton The optimum binding conditions for anti-snag are et al. (1988) could not find any molecules unique to the extremely narrow and greatly dependent on the degree brain song control system of the canary, they concluded of tissue fixation. Probably due to the immunogen used, that the song system must be characterized by a combi- anti-snag preferentially binds to tissue samples that are nation of common molecules with widely overlapping lightly fixed in 2% paraformaldehyde (ph 7.4). Neither patterns of distribution in other unrelated brain areas. unfixed nor overfixed tissue (greater than 40 min expo- In this study, we took a different approach in an at- sure to the fixative) displayed any significant antibody tempt to find a system-specific molecule. Rather than staining. Furthermore, cryostat sections of fresh brain tissue, dried on coverslips, and subsequently fixed for 1 Correspondence: konishim@starbasel.caltech.edu 30 min in 2% paraformaldehyde, also did not bind the

2 Neuron 546 antibody. Apparently, the epitope is irreversibly destroyed when the sections are first dried since sections from properly perfused brain tissue also did not bind the antibody if mounted on slides and left to dry before immunoreacting. Anti-SNAg Robustly Stains the Song System in Adult Male Zebra Finches To determine the specificity, pattern, and variability of anti-snag staining, ten adult male zebra finches ( 100 days old, post-hatching) were randomly selected from a colony cage and perfused with the best limited-fixation protocol (see Experimental Procedures). In all cases, SNAg immunoreactivity (IR) was intense and localized to only certain nuclei within the song system circuit. The staining patterns and relative intensities between labeled areas were the same among all subjects. The lateral magnocellular nucleus of the anterior neostriatum (LMAN) was the most completely and strongly labeled. All of the immunoreactive cells here were confined within the Nissl-defined borders of the nucleus, while most of the surrounding tissue that comprises the rest of the anterior telencephalon was devoid of any labeled cells (Figure 1A). In coronal sections, a thin strip of immunopositive cells was also seen next to LMAN, and we identified this as the medial magnocellular nucleus of the anterior neostriatum (mman). In the posterior portion of the telencephalon, the hyperstriatum ventrale, pars caudale (HVc) or high vocal center (HVC) (Figure 1B), the interfacialis nucleus of the neostriatum (Nif), and the robust nucleus of the archistriatum (RA) were all immunoreactive, but to varying degrees (Figure 2-1). Many of the neurons in HVc and Nif, for example, were strongly labeled in the adult. In RA, however, only a few cells were labeled, and they were widely scattered throughout the nucleus. In all of the other song system areas in the adult male, including area X, the dorsolateral nucleus of the medial thalamus (DLM), and the uvaeform nucleus (Uva), SNAg-IR was undetectable. Also, the hypoglossal nucleus, which contains the vocal neurons that innervate the syrinx (nxiits), did not show any antibody staining (Figure 2-2). The cerebellum (Figure 2-2), midbrain, and hindbrain regions of the brain all showed few, if any labeled cells, except one of the lemniscal nuclei (see below). Retrograde labeling of HVc neurons that send afferents to either area X or RA (Katz and Gurney, 1981), followed by SNAg staining, showed that both populations of HVc projection neurons are labeled with the antibody (Figure 3). This figure also clearly demonstrates that the antigen is localized to the nucleus of those neurons. To determine the extent of anti-snag labeling, lightly fixed tissue samples from the adult male zebra finch Figure 1. Whole Brain Sections Show the Compartmentalization of Antibody Staining within the Song System of the Adult Male Zebra Finch Coronal Sections of an Adult Male Zebra Finch Show the Highly Localized Anti-SNAg Staining in LMAN, mman (A), and HVc (B). Scale bar: 1mm. hatching development in roughly three phases (see Discussion). During this time, nuclear volumes and cell sizes, especially in HVc and RA, change dramatically (Konishi and Akutagawa, 1985). We therefore investi- gated the pattern and relative amounts of SNAg-IR dur- ing these three critical time periods and found a strong correlation between the stage of development and de- gree of antibody labeling. In the first phase of develop- ment (hatching to approximately 35 days old), no SNAg- IR was detected in any song system areas of the male zebra finch (n 5, 20 days old), including LMAN (Figure 4-1: C1 and C2). At the onset of the second phase, around 35 days of age, antibody labeling in the male LMAN, Nif, and RA occurred abruptly, intensely, and simultaneously (n 5). Several birds (n 4) were also examined at slightly younger ages, but in no case did (n 2) spleen, testes, and liver were sectioned at 30 we find evidence of a gradual increase of antibody stain- m and probed with the antibody. In all of these cases, ing over time, or an instance of one song system area no labeling was ever detected (data not shown), indicat- being labeled first before any others. In addition, the ing that SNAg occurs primarily, if not exclusively, in the relative number of anti-snag-labeled cells (determined brain. by the number of labeled nuclei in a 100 field of view, see Experimental Procedures) in HVc and Nif were much The Amount of SNAg Staining Changes during higher in 35- to 40-day-old males in comparison to the the Development of the Male Song System same areas in adult males. The average number of HVclabeled The song system of the male zebra finch is not present cells in 40-day-old males (n 4) was 1310 in its adult form at hatching, but emerges during post- 164 cells/mm 2. By adulthood ( 100 days old, n 5),

3 Song System-Specific Antibody 547 Figure 2. Song System-Specific Staining in the Adult Male Zebra Finch (1, left panel) Various song system nuclei in the adult male zebra finch that stain with anti-snag: (A) HVc, the hyperstriatum ventrale, pars caudale; (B) Nif, the nucleus interfacialis; (C) RA, the robust nucleus of the archistriatum; (D) LMAN, the lateral magnocellular nucleus of the anterior neostriatum. (2, right panel) Other areas of the adult male zebra finch brain, including parts of the song system, that do not show any SNAg labeling: (A) area X; (B) the dorsolateral nucleus of the medial thalamus (DLM), and the uvaeform nucleus below it (Uva); (C) the hypoglossal nucleus (nxiits) that innervates the syrinx; and the cerebellum (Cb). Sagittal sections, anterior is to the right and dorsal side is up. Scale bar: 300 m. and culminating in a highly reduced adult female song system (Konishi and Akutagawa, 1985; Kirn and De- Voogd, 1989; Nordeen et al., 1992). No SNAg staining was detected in any song system areas of 20-day-old females (n 5), 40-day-old (n 5), or adults ( 100 days old, n 5) (Figure 4-1: B1 and B2). However, one of the lemniscal nuclei was antibody labeled in the female, and at all ages studied above. These results suggest that SNAg is either not present in the song system of female zebra finches, or is present in such low quantities that we were not able to detect it. the number of labeled cells dropped to cells/mm 2, and this difference was statistically significant (Student s t test, p 0.001). Likewise, in Nif, the average number of labeled cells in 40-day-old males was cell/mm 2, and dropped to cells/mm 2 in adults (p 0.001). In contrast, the number of labeled cells in LMAN and RA remained relatively constant during these two age periods. In LMAN, the number of labeled cells in 40-day-old males was cells/mm 2. By adulthood, the average number of labeled cells did not change significantly at cells/mm 2 (p 0.569). In RA, the relatively few but widely separated labeled cells also did not show much fluctuation during the later stages of development. In 40-dayold males, the number of labeled cells in RA was only cells/mm 2. In adults, the number of labeled cells was cell/mm 2, and this change was not statistically significant (p 0.229). A summary of the developmental changes in SNAg-IR occurring in the male is presented in Figure 5. SNAg Is Not Detected in the Adult Female Zebra Finch Song System In contrast to the male, the song system of the female zebra finch undergoes a prolonged period of atrophy and cell death, beginning at around days of age, SNAg Expression Is Induced in Female Zebra Finches by Early Estrogen Masculinization While the female song system normally atrophies during development, this can be prevented by administration of estrogen during a critical period early in life (Gurney and Konishi, 1980; Gurney, 1981; Nordeen et al., 1987; Pohl-Apel, 1985; Konishi and Akutagawa, 1988). This hormone treatment not only results in an increased adult song system, containing more and larger neurons comparable in size to males (masculinized), but also produces a functional circuit whereby the hormone-treated female can sing as an adult (Konishi and Gurney, 1982; Pohl-Apel and Sossinka, 1984; Pohl-Apel, 1985; Konishi and Akutagawa, 1987, 1988; Simpson and Vicario,

4 Neuron 548 Figure 3. Both Populations of HVc Projection Neurons Express SNAg HVc neurons in the adult male zebra finch show retrograde labeling by biotinylated dextran amine (brown) and double-labeled with anti-snag. HVc neurons that project to either (A) RA or (B) area X are both labeled with the antibody (black, nuclear staining). Scale bar: 10 m. 1991a, 1991b). Untreated female finches never sing, and atrophied song system (Arnold, 1974; Konishi and Akutagawa, hormone administration in adult females does not induce 1988). Moreover, female chicks that have been singing, nor does it affect the morphology of the estrogen treated develop their masculinized song sys- Figure 4. Comparisons of the Different Types of SNAg Staining Occurring in LMAN Neutral red and anti-snag double-stained sections demonstrate the specificity, sexual dimorphism, and developmental regulation of SNAg. Each panel shows a low (left) and high power (right) view of LMAN. Panel 1: (A1, A2), adult male zebra finch; (B1, B2), adult female zebra finch; (C1, C2) 20-day-old male zebra finch. Panel 2: (A1, A2), adult male canary; (B1, B2), adult male Bengalese finch; (C1, C2), estrogenmasculinized female zebra finch. Scale bar: 200 m (low power, left panel), 10 m (high power, right panel).

5 Song System-Specific Antibody 549 nor novel stained areas in the hormone-treated female. Consistent with the idea of a critical period for hormone action, estrogen treatment of normal adult females (n 4) failed to masculinize any part of the atrophied song system, and also did not induce any SNAg expression (data not shown). Several female zebra finches (n 2) were also treated with estrogen at hatching, and then their hormone pellets were removed at 15 days of age. Previous experience has shown that the pellet needs to be in place for at least 15 days in order to fully masculinize the female s song system (our unpublished observations, but see also Pohl-Apel and Sossinka, 1984; Adkins- Regan and Ascenzi, 1987). However, this manipulation did not alter either the timing of the song system differentiation or the age of the onset of SNAg-IR. In males, neither exogenous estrogen (n 3) or testosterone (n 4) in hatchlings nor bilateral castrations at 15 days of age (n 3) altered either the timing or relative amounts of SNAg-IR that occur later in development. Figure 5. Developmental Regulation of SNAg Expression in Various Song System Nuclei of the Male Zebra Finch (Top) Schematic representation of the adult male zebra finch brain and the song system areas that are stained with anti-snag. Not all song system nuclei are labeled with the antibody. (Bottom) Developmental regulation of SNAg expression throughout the three phases of development in a male zebra finch. The amount of SNAg-IR peaks during approximately days of age, but only in HVc and Nif. Asterisks above the adult HVc and Nif bars denote a significant change from the 40-day-old values. tems over approximately the same time course as normal males (Konishi and Akutagawa, 1988). To determine whether SNAg could be induced in the female brain, we treated female hatchlings (n 6) with estrogen pellets (50 g, s.c.) and examined them during development into adulthood for SNAg-IR. Our results show that adult female zebra finches that were estrogen treated as chicks show the same pattern of intense and highly localized immunoreactivity seen in adult males. Like the male, LMAN was the most completely and strongly labeled brain area (Figure 4-2: C1 and C2). Similarly, HVc, Nif, and RA all showed SNAg-IR, although the proportion of labeled cells in each of these brain areas was significantly smaller than the corresponding areas in the adult male brain (compare the male LMAN in Figure 4-1: A1, with the E2 female LMAN in Figure 4-2: C1). Furthermore, the onset of antibody labeling in the hormone-treated female appeared to be rather sudden, at around 35 days of age, which is very similar to the timing in the male. When compared to the male, there were neither atypical Taxonomic Differences in Anti-SNAg Staining We investigated whether SNAg expression occurs primarily in songbird species like zebra finches that possess a highly dimorphic song system. In two other species of estrildine finches, the Bengalese finch (Lonchura striata) and the spice finch (Lonchura punctulata), their adult song systems are also highly sexually dimorphic. The antibody labeling pattern in both of these species was nearly identical to that in the zebra finch. Very strong and highly localized antibody labeling occurred in LMAN (Figure 4-2: B1 and B2), Nif, HVc, and RA of the adult males (n 5), whereas these same areas in the adult female (n 5) showed no labeled cells at all. The same hierarchy of labeled song system areas was also present in these birds, with LMAN showing the largest, and RA the smallest, proportions of immunoreactive cells. Like the zebra finch staining pattern, the cerebellum, midbrain, and hindbrain regions all showed very little, if any, labeled cells. There were no new areas in these other songbird types that showed labeled cells that differed with the male zebra finch staining pattern. In addition, there are some estrildine finches in which both the adult male and female can sing, and there are few gender differences between the sizes of their song systems. In such birds, like the strawberry finch (Amandava amandava), intense antibody staining occurs in the song system of both sexes (n 2, each sex). The staining pattern is nearly identical in these birds as in the zebra finch adult male, and LMAN again shows the highest amount of SNAg-IR (Figures 6A and 6B). In other songbirds, such as the canary (Serinus canaria, family Emberizidae), gender differences in the adult song system are not nearly as extreme as those seen in the adult zebra finch brain (Brenowitz and Arnold, 1986; DeVoogd, et al., 1988). Yet, despite the presence of a large and well-developed song system in the adult brains of both sexes (n 2 adult males, 1 adult female), very little, if any, antibody staining could be detected in any song nuclei, including LMAN (Figure 4-2: A1 and A2). Antibody staining was also attempted on brain sec- tions from adult male and female rats (n 1, each sex). Particular attention was focused on the sexually dimor- phic region of the pre-optic area (Gorski et al., 1978), but

6 Neuron 550 Figure 6. SNAg Expression Is Not Necessarily Gender Specific nor Exclusively within the Song System Anti-SNAg labels LMAN cells in both the adult male (A) and female (B) strawberry finch (Amandava amandava). Anterior is to the right, and dorsal side is up. (C), coronal section showing SNAg labeling in the lemniscal nucleus of a 20-day-old male zebra finch. (D), coronal section showing SNAg labeling in the lemniscal nucleus of an adult female zebra finch. Medial is to the right, and dorsal side is up. Scale bar: 300 m. anti-snag did not produce any labeled cells anywhere in eration. This also provided a convenient internal control, the rodent brain (data not shown). whereby any novel band(s) that may be precipitated should reflect the fixation test results seen on vibratomecut sections. In this experiment, vibratome sections of SNAg Is Highly Specific but Not Exclusive to the Song System fresh, unfixed adult male zebra finches were first sub- While the vast majority of antibody-labeled cells were jected to varying fixation conditions before reacting restricted to only a few but well-demarcated areas of the them with anti-snag (see Experimental Procedures). In song system, a few labeled cells could also be detected this way, the optimum binding conditions for the antioutside of it. The occurrence and location of these laplied toward the immunoprecipitation experiments. The body were determined (Figure 7A), and the results ap- beled cells were highly variable from bird to bird, were results revealed a band whose emergence was both usually only weakly immunoreactive, and they were typifixation dependent and tissue specific (Figure 7B). Simically very widely scattered. The notable exception was lar to the results on fixed tissue sections, the intensities one of the lemniscal nuclei in the hindbrain, which we of the band varied with the fixation of the homogenate, tentatively identified as the nucleus ventralis lemnisci with the highest antibody-antigen affinity obtained after interalis, pars anterior (VLVa). This area is not part of 30 min of exposure to 2% paraformaldehyde. This is in the song control pathway, but was nevertheless labeled good agreement with the results of fixation tests on by anti-snag at any age, and in both sexes (Figures 6C vibratome-sectioned tissue (compare Figures 7A and and 6D). 7B). Because the antigen is fixed prior to electrophoresis, the location of the band may not be indicative of its SNAg Can Be Immunoprecipitated true molecular weight since any accurate determina- Homogenates of both adult male zebra finch LMAN and tions must be done on unfixed, denatured protein (Towcerebellar tissue were immunoprecipitated with anti- bin et al., 1979; Burnette, 1981). SNAg. Because of the stringent requirements necessary for antibody binding to take place on fixed tissue sec- SNAg s Epitope Is on a Core Peptide tions, the immunoprecipitation experiments were de- Enzymatic digestions of fixed tissue sections (see Experimental signed to take the degree of fixation into special consid- Procedures) using alkaline phosphatase or

7 Song System-Specific Antibody 551 in less than 2 min at 37 C. Even at concentrations as low as 25 g/ml, the sections were rendered too fragile to handle after only 10 min of incubation time. However, using a control monoclonal antibody against a phosphorylated neurofilament epitope (Zymed) did show diminution of immunostaining on our sections after 3 hr of alkaline phosphatase digestion without prior treatment with trypsin. Western Blots Homogenates of adult male zebra finch telencephalon, adult female telencephalon, or cerebellum tissue all failed to show any specific staining on conventional Western blots (data not shown). Attempts to fix either the homogenates, acrylamide gels, or nitrocellulose filters after transfer of the proteins were also unsuccessful and probably reflect the highly stringent fixation conditions for the antibody to bind to SNAg. Discussion This paper shows the existence of an antigen whose distribution is highly specific to a discrete set of interconnected brain areas that controls a specific behavior. Previous studies have shown the expression and accumulation of various molecules in the song system of male zebra finches, including synelfin (George et al., 1995), brain-derived neurotrophic factor (Akutagawa and Konishi, 1998), ZENK (Jarvis and Nottebohm, 1997), c-fos (Kimpo and Doupe, 1997), and CGRP (Bottjer et al., 1997), to name a few. However, antibody labeling in all of these cases also included many other unrelated brain areas and was simply more concentrated in some song nuclei, for example, during singing (Jarvis et al., 1998) or while hearing conspecific song (Sakaguchi et al., 1999). In contrast, our antibody identifies a molecule that is even more restricted to the zebra finch song system than any other markers previously described. At present, the functional significance of SNAg is unknown, but the timing and pattern of its expression, taken to- gether in context of the differentiation of the song system, is worth elaborating. Figure 7. SNAg Can Be Immunoprecipitated when Properly Fixed (A) Antibody staining is dependent on the degree of tissue fixation. The optimum binding condition of anti-snag (circular area) was determined by varying either the time or the strength of the fixative. Symbols:, strong antibody labeling;, weak labeling;, label barely detectable;, no label. (B) Immunoprecipitation panel of tissue homogenates prepared from the lateral magnocellular nucleus of the anterior neostriatum (M) or cerebellum (Cb) reacted with anti-snag shows a novel band ( ) whose occurrence is both fixation dependent and tissue specific. The time values above each of the column sets (0,5,.etc.) represent the different amount of minutes the homogenate pairs were exposed to the fixative before the addition of the antibody (see Experimental Procedures). Molecular weight markers are in kilodal- tons, and are used for relative and not quantitative purposes (see text). peptide-n-glycosidase F failed to diminish immunostaining with anti-snag (data not shown). Even with very high concentrations of the enzyme ( 67 U/ml calf intestinal alkaline phosphatase, 6 U/ml PNGase F), or prolonged incubation times (up to 3 days), the staining intensities of test sections were just as strong to sideby-side control sections that did not receive any enzyme treatment. Prior treatment of the sections with dilute trypsin (400 g/ml), which has been reported to facilitate alkaline phosphatase digestion (Sternberger and Sternberger, 1983) in paraffin-embedded, Bouin-fixed sections, completely digested our partially fixed sections The Onset of SNAg Staining Coincides with the Differentiation of the Song System The abrupt expression of SNAg in the song system of both males and estrogen-treated females at around 35 days of age suggests that a sudden change is occurring in their brains at that point in development. In fact, several critical changes are known to occur during this time period. Until about 35 days after hatching, both male and female zebra finch chicks possess a song system whose neurons are still somewhat small in size and densely packed within their respective nuclei, with the exception of the female s area X, which is not recognizable. At around days of age, the male song system rapidly grows in nuclear volume, interneuronal spacing, and soma size, especially in HVc and RA. During this same period, the corresponding areas in the female brain undergo gradual atrophy and cell death (Konishi and Akutagawa, 1985; Kirn and DeVoogd, 1989; Nordeen et al., 1992). The end result is a sexually dimorphic adult brain (Nottebohm and Arnold, 1976). However, ad-

8 Neuron 552 ministration of estrogen to female chicks any time be- ministered during a critical period of early development fore, but not after, approximately 45 days of age will not and has no effect on adult females (Konishi and Akuta- only prevent the massive cell death that normally occurs gawa, 1988). If the neurons destined to express SNAg in their song systems, but will also promote the later die in females, sexual dimorphism in antigen distribution differentiation of those saved neurons (Gurney and results. While many neurons do die in the normal female Konishi, 1980; Gurney, 1981; Nordeen et al., 1987; Pohl- forebrain song nuclei, a significant number survive in Apel, 1985; Konishi and Akutagawa, 1988). As a result LMAN and can be stained by retrograde transport from of the early hormone treatment, the female song system RA (Nordeen, et al., 1992). Although these neurons have develops in much the same way and timing as the male, distinct nuclei, they still do not express SNAg (Figure and the adult female can also sing (Konishi and Gurney, 4-1: B2). In males, previous experiments have shown 1982; Pohl-Apel and Sossinka, 1984; Pohl-Apel, 1985; that neither early castration (Arnold, 1975; Adkins- Konishi and Akutagawa, 1987, 1988; Simpson and Vicario, Regan and Ascenzi, 1990) nor early treatments with exogedays 1991a, 1991b.). Furthermore, at approximately 35 nous estrogen or testosterone (Gurney, 1981; Schlinger post-hatching, HVc in males makes axonal connections and Arnold, 1991; Wade et al., 1997) significantly alters to one of its afferent targets, RA. Prior to this the development of the song system. Since our results age, LMAN axons have already made connections to show that the amount and timing of SNAg-IR is also not RA neurons in both sexes (Nordeen et al., 1992; Mooney affected by these manipulations, they suggest that the and Rao, 1994), but the terminals of HVc axons lie just appearance of SNAg is more closely associated with the outside of and dorsal to RA. At around 35 days of age, differentiaton of the song system, rather than through a HVc axons suddenly innervate RA. In the normal female, direct interaction with the hormone. Furthermore, while this HVc to RA connection is never made, but in estrogen exogenous estrogen effectively masculinizes the females pretreated females, the occurrence and timing of this song system, and Holloway and Clayton (2001) crucial connection closely parallel that of the male (Koni- have demonstrated in vitro that estrogen synthesis in shi and Akutagawa, 1985, 1987; Simpson and Vicario, the male brain is responsible for its song system differ- 1991b). In addition, Holloway and Clayton (2001) have entiation, very few estrogen receptors are actually located demonstrated in vitro that estrogen synthesized in the in any of the forebrain song nuclei (Gahr et al., male s brain is responsible for triggering the formation 1987; Gahr and Konishi, 1988). Taken together, these of this important synaptic connection. During the ontogeny observations support the view of an indirect effect of estro- of the male song system, LMAN and HVc exert gen on the differentiation of the song system and the different but important afferent influences on the normal concomitant expression of SNAg (Gahr and Konishi, development of RA (Akutagawa and Konishi, 1994). 1988; Bottjer and Johnson, 1992). Thus, several important developmental processes take place in both males and estrogen-treated females during SNAg Is Not a Known Hormone Receptor a rather restricted period. It is at the onset of this phase The appearance of SNAg at the onset of sexual differenof neuronal development when anti-snag labeling sud- tiation may indicate its role in neuroendocrine prodenly occurs in the song system. Into adulthood, there cesses. The antigen does not, however, appear to be a is a gradual decrease in nuclear volume and cell size, known steroid hormone receptor, as the distribution of especially in HVc (Konishi and Akutagawa, 1988), and these receptors in the brain is not consistent with that there is also a corresponding decrease in the amount of SNAg. In zebra finches, estrogen receptors occur only of SNAg-IR (Figure 5). Since the results of the enzyme in a part of HVc and its vicinity within the song control digestion experiments indicate that the epitope is on a system and in various hypothalamic areas (Gahr, et al., core peptide, it seems unlikely that the sudden SNAg 1987; Gahr and Konishi, 1988). Although androgen restaining pattern is associated with a posttranslational ceptors occur in most of the forebrain song nuclei of modification of an existing protein as it matures into the male zebra finch, they also occur elsewhere in the part of a functional pathway. Also, since the lemniscal brain (Arnold and Saltiel, 1979; Balthazart et al., 1992). nucleus is always stained by anti-snag at any age and Furthermore, these receptors are found in the forebrain sex, it serves as a good internal control that insures us song nuclei of other species such as the canary, where that our immunohistochemical procedures are always SNAg is not detected. in check (Figures 6C and 6D). This is especially important when we failed to detect any labeled cells in the song Not All Songbird Species Express SNAg systems of 20-day-old males. in Their Song Systems The differential expression of SNAg among songbirds Estrogen Induces SNAg Expression in the Female is intriguing. While many monoclonal antibodies can be Song System very species specific, our observation that anti-snag The relationship between gender, estrogen, and SNAg did not stain any song system areas of adult male or expression also calls for closer examination. Exogenous female canaries is surprising (Figure 4-2: A1 and A2) estrogen enables female neurons to express SNAg at since canaries and zebra finches both have very similar the appropriate time and place. This effect occurs even brain anatomies. It s possible that SNAg is present in if the hormone pellet is removed 20 days before the the canary song system, but at such low levels that we onset of masculine differentiation. Thus, estrogen may could not detect it reliably with the antibody. Since we prevent SNAg-expressing song neurons from dying, or did not investigate juvenile canaries, it is also possible it may induce SNAg expression as part of the male differ- that SNAg is expressed in relatively larger amounts during entiation program. However, the hormone must be ad- this stage of development (as in zebra finches) and

9 Song System-Specific Antibody 553 declines to undetectable levels in the adult bird. However, Experimental Procedures since the intense SNAg-IR that occurs in the zebra Antibody Production finch LMAN does not fluctuate very much from 35 days Fresh whole brains from 50 adult male zebra finches (Taeniopygia into adulthood (Figure 5), either a different expression guttata) were immersion fixed in 2% w/v paraformaldehyde in 25 pattern for this song nucleus exists in canaries, or SNAg mm phosphate buffer (PB), ph 7.4, at 4 C for 2 5 weeks. The fixed is simply absent. Since canaries and zebra finches bedissected brains were sectioned on a vibratome (200 m), and RA was carefully long to different families, SNAg may be unique to family out and washed in three changes of cold PB over 1 hr. Estrildidae. Ultimately, more sensitive techniques such The RA tissue was combined and homogenized using a Polytron homogenizer in 25 mm PB at a concentration of 75 mg/ml. Antibodas Western blots or in situ hybridizations will be necesies were produced following the cyclosphosphamide immunosupsary to confirm SNAg s presence or absence in other pression approach (Matthew and Sandrock, 1987; Ou et al., 1991). avian species. Also, the presence of SNAg in the lemnis- Immune suppression homogenate was prepared from similarly fixed cal nucleus of the female zebra finch indicates that it is cerebellum tissue, also at approximately 75 mg/ml. Balb/c male not completely unique to the male brain. In the straw- mice, 4 6 weeks old, were immunized with three daily injections berry finch, both sexes have well-developed song sysphamide (0.5 ml) injected interperitoneally, followed by injection of cyclophos- (120 mg/kg). Suppression injections were repeated twice tems, both can sing, and both express SNAg in their at two week intervals. Two injections of RA homogenate were given song systems equally well (Figures 6A and 6B). two weeks after the last suppression, and spleen cells for fusion were harvested four days later. Supernatants from the resulting hybridomas were tested on 30 m thick sections of immersion-fixed SNAg Is a Molecular Signature of a Specific adult male zebra finch brains that were cut on a freezing microtome. Brain Circuit The ability of the brain to produce coherent, coordinated Antibody Binding Conditions The optimum binding conditions of the antibody were determined behavior relies on the precise formation and functioning using vibratome sections of fresh, unfixed male zebra finch brains. of discrete neural pathways. The development of such In the first study, sections (50 m) were placed in varying strengths circuits could be guided by particular combinations of of fixative (paraformaldehyde), keeping the time of exposure concommon chemical markers (Sutcliffe et al., 1983; Gold- stant at 30 min. The fixative was immediately washed out of the man et al., 1986; Uhl and Sasek, 1986; Branks and Wilthe sections in cold 25 mm PB, and immunoreacted with anti-snag. In son, 1986; Clayton et al., 1988), or by the patterned second study, the fixative strength of 2% paraformaldehyde was kept constant, while the exposure time to the fixative was expression of unique molecules (Levitt, 1984; Zacco et varied. al., 1990; Pimenta et al., 1995; Reinoso et al., 1996). The existence of such restricted molecular signatures was Tissue Preparation and Immunohistochemistry first hinted at when monoclonal antibodies were gener- A breeding colony in our animal facility provided us with both adult ated against the leech nervous system (Zipser and zebra finches ( 100 days old) and young birds of known ages. McKay, 1981). While these antibodies revealed some Experimental birds were lethally anesthetized and transcardially perdifferential staining between functional compartments, fused with 0.9% saline, followed by 2% paraformaldehyde in 25 mm there was also considerable overlap with other unrelated phosphate buffer (ph 7.4) for 30 min at room temperature. Excess fixative was immediately washed out by changing the perfusate to areas. Since then, several other molecules have been 25 mm PB-10% sucrose, and the perfusion continued for an addishown to be expressed in distinct neural systems, such tional 45 min. The brain was immediately removed from the skull, as bodenin (Faisst and Gruss, 1998), neurotrimin (Gil et the dura taken off, and cryoprotected by immersing it in cold PBal., 1998), cad-6 (Inoue et al., 1998), and the IgLON family 30% sucrose overnight. The following morning, 30 m sections were or neural cell adhesion molecules (Pimenta et al., 1995; cut on a freezing microtome and collected in cold PB. Struyk et al., 1995; Zhukareva and Levitt, 1995), of which For immunohistochemistry, the sections were washed in 25 mm PB, blocked for 1 hr in 5% normal goat serum in PB containing the limbic system-associated membrane protein (LAMP) 0.1% Triton X-100 (Sigma). Antibody supernatants were incubated shows the most system-specific staining pattern. How- with test sections at room temperature for 1 hr and washed for an ever, there are some exceptions to the limbic system additional hour in three changes of PB-0.1% Triton. A secondary, identity of LAMP expression, and the exact definition of anti-mouse IgM conjugated to horseradish peroxidase (Boehringer- what areas of the brain actually comprise the limbic Mannheim/Roche) was then applied for 1 hr at a concentration of system remains vague and is periodically updated (Lev- 8 g/ml and subsequently washed in three changes of PB-0.1% Triton for 1 hr. Horseradish peroxidase activity was visualized by itt, 1984; Reinoso et al., 1996). While just a few molecules reacting the sections for 3 min in 0.02% diaminobenzidine (DAB, have been shown to be specific to a given circuit in the Sigma) and 0.009% hydrogen peroxide in 25 mm PB (ph 7.4). The brain, they all share the same properties: their expres- antibody-labeled sections were then washed in PB and either sion peaks during a particular time of neural developmetal mounted onto subbed slides and coverslipped, or were first heavy ment, and more importantly, they have all been shown intensified with nickel-cobalt (Adams, 1981), and then mounted and counterstained with a 1% solution of neutral red to be involved in selective neuronal growth, axon tar- (Sigma). Control sections were also processed with the primary geting, or synapse formation during the differentiation antibody omitted, resulting in no labeled cells anywhere. of their respective pathways (Zacco et al., 1990; Inoue et al., 1998; Faisst and Gruss, 1998; Gil et al., 1998). In Quantification of Anti-SNAg-Labeled Cells in Various Song this study, we document a molecule whose system- System Nuclei by Age Group specific expression pattern rivals that of LAMP. In addi- Antibody-labeled tissue sections (30 m) from HVc, RA, Nif, and tion, the song system of the zebra finch is more clearly LMAN were observed through a light microscope (Nikon DX) using a 100 oil immersion objective. Ten random samples, taken over defined than the limbic system in rats, both anatomically the entire extent of each song nucleus, were used to count the total and behaviorally. Further progress in identifying SNAg number of SNAg-IR cells in the 100 field of view (f.o.v.). Since the is encouraged by its isolation and partial purification antibody staining is restricted to the nucleus of neurons, stereological through immunoprecipitation experiments (Figure 7). methods were considered unnecessary, as any labeled nuclei

10 Neuron 554 that were split during sectioning were obvious and were discarded pended in 200 l of PB. Electrophoresis bromphenol blue dye mixture from quantification. Only intact, fully stained nuclei were counted, was added to each sample, and then heated to 85 C for 10 and any fragmented or unevenly stained nuclei were ignored. The min. After 10 min of centrifugation, equal volumes of the resultant area of the f.o.v. was calculated by first drawing the perimeter onto supernatants were run on a 7.5% SDA-polyacrylamide gel for paper using a drawing tube attached to the microscope and measur- analysis. ing the area of the resultant circle using a computer-assisted digitizing tablet. The average number of SNAg-IR cells from each brain area and age were calculated by dividing the mean number of SNAg- Enzymatic Digestions IR cells by the area (mm 2 ) of the f.o.v. A computer software program Fixed tissue sections (30 m) were treated with calf intestinal alkawas then used to conduct a two-way analysis of variance (Student s line phosphatase (Roche) at 400 g/ml (68 U) in 0.05 M Tris-HCl, t test) between the number of SNAg-IR cells in each brain area and ph 7.5, 0.3 M NaCl for hr at 37 C (Sternberger and Sternberger, the corresponding post-hatching age. 1983). Control sections were also incubated in the same solutions, but without the addition of the enzyme. Subsequently, the sections were washed three times over 1 hr in TBS, blocked for 1 hr in TBS Double Labeling HVc Projection Neurons 0.1% BSA-0.1% Triton 100, then stained with anti-snag as usual. To determine whether the HVc cells that express SNAg project to Similarly, another set of fixed sections was treated with peptide-n- RA, area X, or both, we double stained these projection neurons by glycosidase F (Sigma) at 6 U/ml (Saez, et al., 2000), in 0.1 M Trisretrograde labeling followed by anti-snag immunohistochemistry. HCl, ph 8.0, 150 mm NaCl, 2.5 mm EDTA, at 37 C for 24 hr, then Biotinylated dextran-amine (BDA) was iontophoretically delivered stained with anti-snag. Trypsin (Sigma) pretreatment of fixed tissue to either RA or area X by a combination of stereotaxic coordinates sections was also attempted with serial dilutions ranging from 400 and their characteristic spontaneous activities by electrophysiologi- g/ml down to 25 g/ml in 0.05 M Tris-HCl, ph 7.6, 0.3 M NaCl for 10 cal recordings. A 10% (wt/vol) solution of BDA (Molecular Probes) min at 37 C. As a positive control, an anti-neurofilament monoclonal in 0.2 M KCl was iontophoresed through a finely pulled capillary antibody (Zymed) against a phosphorylated epitope was used on glass electrode (15 m tip diameter) for 10 min (RA) or 20 min (area our partially fixed, untrypsin-treated sections either before or after X) using a 10 A positive current pulsed in 7 s intervals. After a 2 alkaline phosphatase treatment. After 3 hr of incubation, staining day survival period, the birds were perfused with the best limitedintensity with the antibody was noticeably decreased (data not fixation protocol (see previous Experimental Procedures). The folshown). lowing day, the brains were sectioned on a freezing microtome (30 m) and stored in cold PB. Anti-SNAg immunohistochemistry was performed first, using the cobalt intensification protocol (Adams, Western Blots 1981) to turn the reaction substrate dark blue. After three washes Brain homogenates were prepared from adult male zebra finches, in 25 mm PB, the SNAg-labeled sections were processed with the adult female zebra finches, and cerebellum tissue from the adults Vector Elite kit (Vector Labs), washed thoroughly, and reacted with of either sex.. The birds (n 2, each sex) were decapitated, the 0.02% DAB and 0.009% hydrogen peroxide in PB. The resulting telencephalons or cerebella removed, and Polytron-homogenized retrograde BDA in the cytoplasm was rendered brown in color by at 4 C in a homogenizing fluid (Maniatis et al., 1989): 50 mm Tristhis procedure and contrasted well with the dark blue/black precipi- HCl, ph 8.0, 150 mm NaCl, 0.02% sodium azide, 0.1% SDS, 100 tate of the SNAg-labeled neuronal nuclei. Cells were considered g/ml PMSF, 1 g/ml aprotinin (Sigma), 1% Nonidet P-40, and 0.5% double stained only when both labels were clearly colocalized within sodium deoxycholate. After homogenization, the samples were sonthe same neuron and in the same focal plane. icated and centrifuged at 12,000 g for 20 min at 4 C. The supernatants were transferred to a clean tube and the total protein in each Immunoprecipitation of SNAg sample preparation was quantified using a Beckman spectropho- Fresh LMAN and cerebellum (Cb) tissue were carefully excised from tometer and BioRad s Protein Quantification kit. Each sample prepaapproximately 50 adult male zebra finches and stored immediately ration was concentration matched and 75 g of each homogenate in separate vials in liquid nitrogen after each dissection. Approxi- was loaded, electrophoresed on a 7.5% SDS-polyacrylamide gel, mately 0.5 g of each tissue type was homogenized (using a Polytron and transferred to a nitrocellulose filter. Subsequently, the nitrocelhomogenizer) in 10 ml of a solution consisting of 50 mm Tris-HCl lulose was blocked for 1 hr in 25 mm PB, 1 casein solution (Vector (ph 8.0), 150 mm NaCl, 0.02% sodium azide, 0.1% SDS, 100 g/ml Labs), 0.1% Tween-20, then incubated in the anti-snag supernatant PMSF, 1 g/ml aprotinin, 1 g/ml leupeptin, 0.5% turkey egg-white overnight at 4 C. After rinsing over 30 min in PB-Tween-20, the trypsin inhibitor, 1% NP-40, and 0.5% sodium-deoxycholate. The nitrocellulose was incubated for 1 hr in a biotinylated anti-mouse homogenates were briefly sonicated and centrifuged at 12,000 g IgM (1.5 g/ml, Vector Labs), washed again, then treated with the for 30 min at 4 C. Supernatants were carefully removed and the Vectastain ABC-AmP reagents. Substrate development with either volumes were adjusted so that both the LMAN and Cb homogenates NBT/BCIP for alkaline phosphatase or chemiluminescence with Duowere concentration matched to approximately 1 mg/ml, using a Lux (Vector) both failed to produce any specific band(s) in the male Beckman spectrophotometer and a BioRad protein quantification brain, as compared to either the female or cerebellum tissue. In kit. Two milliliters of the LMAN and Cb homogenates were removed addition, other modifications were attempted where either the hoand set aside as 0-fixed samples. To the remaining 10 ml of fluid, mogenate, polyacrylamide gel, or nitrocellulose after the transfer 10 ml of freshly prepared 4% paraformaldehyde in 25 mm PB was was fixed in 2% paraformaldehyde in 25 mm PB for 30 min, washed, added slowly with constant stirring. At intervals of 5, 15, 30, 45, and then incubated with the antibody. As before, no specific signal was 60 min, 2 ml aliquots were removed and immediately diluted with ever detected on the Western blots. 18 ml of cold 25 mm PB. The 0-fixed sample was also diluted in the same way. All homogenate samples were subsequently dialyzed overnight in 25 mm PB containing 0.05% sodium azide. The follow- Estrogen-Treated Females ing day, a 10 ml sample was transferred to a beaker, and 200 l 17- Estradiol (Sigma) was mixed with silastic rubber (Dow Corning of anti-snag supernatant was added with gentle stirring for 1 hr, RTV-738) in a ratio of 1 part hormone to 11 parts silastic (Gurney, followed by the addition of 50 l of rabbit, anti-mouse IgM (Zymed, 1981). The slurry was back loaded into a 1 ml disposable syringe 0.5 mg/ml) for an additional hour. After incubation with the second- and extruded in ropes onto a glass slide through a 19G needle. ary antibody, 100 l of Protein A-positive staphylococcus aureus The ropes were allowed to cure to 2 3 weeks in a 37 C oven, and cells (Cowan I strain, Boehringer-Mannheim/Roche) was added and cut into 1 mm lengths (approximately 50 g estrogen). The estrogen gently mixed for 1 hr. The mixture was then separated on a clinical pellets were implanted subcutaneously over the breast muscle in centrifuge for 5 min at high speed, decanted, and washed three female zebra finch hatchlings. The presence of the pellet was always times in 25 mm PB-0.5% Tween-20 over 30 min. After the last wash, confirmed in the adult birds before any experimental procedures the pellet was suspended in 1 ml of PB-Tween-20, and transferred were pursued. All experiments were performed under the guidelines to a 1.5 ml Eppendorf tube and centrifuged at 12,000 g for 10 established by NIH under the auspices of the Laboratory Animal min. The supernatant was discarded, and the final pellet was resus- Resource Facility of the California Institute of Technology.